Hoisting mechanism for ascending and descending a cable

ABSTRACT

A novel hoisting mechanism for ascending and descending a freely hanging, single strand of cable that includes a dual drive sheave-pressure wheel assembly combination which provides the safety of two separate drive sheaves and the threading simplicity of an open breach, and that includes a brake which severely limits downward drift once power is cut off on descending the cable. The preferred embodiment provides a primary drive sheave and a secondary drive sheave located on axes parallel one to the other and driven through a single, main drive shaft. A pressure wheel assembly cooperates with a tension sheave mounted on the primary sheave&#39;&#39;s drive shaft, the assembly including a series of pressure wheels movable between a first position where same create an open breach between the primary and secondary drive sheaves so as to allow easy threading and unthreading of the cable with the mechanism and a second position where same are spring biased against the cable wound around the tension sheave to prevent slippage of the cable relative to the tension sheave as the mechanism ascends and descends the cable. A special magnetic brake is interconnected with the main drive shaft, the brake being energized when power is cut off on descending the cable so as to severely limit drift of the mechanism down the cable.

United States Patent [191 Mauldin J [111 amazes [451 Feb. 26, 19745 1 norsrmo MECHANISM FOR, ASCENDING AND DESCENDING'A CABLE William E. Mauldin, Cincinnati,

Ohio

[73] Assignee: Hi-Lo Powered Stirrups, Inc.,

Cincinnati, Ohio [22] Filed: Mar. 9, 1972 [21] Appl. No.: 233,019

[75] Inventor:

[52] US. Cl. 254/185 R, 188/171, 254/150 R [51] Int. Cl. 366d 1/30 I [58] Field of Search 254/185, 150; 188/171; 187/73 [56] References Cited UNITED STATES PATENTS 2,938,707 5/1960 Allenbaugh 254/150 R 1,682,083 8/1928 Johnston 254/185 R 2,819,789 l/l958 Lang 254/167 X 3,028,512 4/1962 -Sorchy 188/171 X 3,276,745 Mauldin I Primary Examiner-Evon C. Blunk Assistant Examiner-Johnny D. Cherry- Attorney, Agent, or Firm-Wood, Herron & Evans [57] ABSTRACT A novel hoisting mechanism for ascending and de- I 254/150 R Wendler 188/171 X scending a freely hanging, single strand of cable that includes a dual drive sheave-pressure wheel assembly combination which provides the safety of two separate drive sheaves and the threading simplicity of an open breach, and that includes a brake which severely limits downward drift once power is cut off on descending the cable. The preferred embodiment provides a primary drive sheave and a secondary drive sheave located on axes parallel one to the other and driven through a single, main drive shaft. A pressure wheelassembly cooperates with a tension sheave mounted on the primary sheaves drive shaft, the assembly including a series of pressure wheels movable between a first position where same create an open breach between the primary and secondary drive sheaves so as to allow easy threading and unthreading of .the cable with the mechanism and a second position where same are spring biased against the cable wound around the tension sheave to prevent slippage of the cable relative to the tension sheave as the mechanism ascends and descends the cable. A special magnetic brake is interconnected with the main drive shaft, the brake being energized when power is cut 011' on descending the cable so as to severely limit drift of the mechanism down the cable.

2 Claims, 5 Drawing Figures PATENTED H58 2 s 1974 SHEEI 2 OF 2 l l l l l HOISTING MECHANISM FOR ASCENDING AND DESCENDING A CABLE This invention relates to hoisting mechanisms and, more particularly, relates to hoisting mechanisms especially adapted for ascending and descending a freely hanging, single strand of cable.

Swing stage or free hanging scaffolds are customarily used in construction, repair and painting work for raising and lowering workers along the face of a building or the like. More recently, such swing stage scaffolds have found wide use in connection with washing windows'of high-rise or skyscraper type buildings. In such instances a scaffold may be supported between two hoisting mechanisms, i.e., a hoisting mechanism is positioned at each end of the scaffold, and the mechanisms operated in tandem to raise and lower the scaffold.

Also, during construction of a building elevator cages are customarily used for movingv men and materials from ground level to upper floors of that building. A single hoisting mechanism is often used for raising and lowering such elevator cages. r

In the case of swing stage scaffolds and/or elevator cages adapted for use with suchjhoisting mechanisms, the hoisting mechanism may be of the type having' a power driven sheave or pulley system through which a freely hanging, single strand of cable is threaded. This cable is suspended from an overhead support. When the mechanisms sheave or pulley system is driven in one direction the mechanism climbs the cable, and when the mechanisms sheave system is driven in the opposite direction the mechanism descends the cable.

Basically, there are two types of hoisting mechanisms adapted for use with freely hanging, single. strands of cable. These two basic types are an accumulating cable system and a non-accumulating cable system. In an acsimply threaded into the hoisting mechanism and extends at all times from the mechanism to ground level no matter what the mechanisms position relative to ground level; the hoisting mechanism simply climbs up and down the cable and does not accumulate the cable inside an enclosure as it ascends the cable, i.e., the

cable hangs freely beneath the mechanism at all times as it ascends and descends the cable.

Power driven hoisting mechanisms of the nonaccumulating type particularly adapted for ascending and descending a. freely hanging, single strand of cable are well known to the prior art. Such hoisting mechanisms are shown, for example, in Mauldin U.S. Pat. No. 3,276,745; Allenbaugh US. Pat. No. 2,938,707, and Allenbaugh U.S. Pat. No. 2,662,734; these hoisting mechanisms are all particularly adapted for use with swing stage scaffolds and elevator cages of the type described above. The hoisting mechanism structures taught by these patents have proved relatively successful from a commercial standpoint over the years. It is believed one reason for such success is that each can be adjusted to provide an open breach so that the sheave or pulley system can be easily threaded or un-. threaded with the single strand of cable when the hoisting mechanism is resting on the ground/Such an open breach of the sheave system is desirable'because the cable can be threaded into the sheave system at' any point intermediate its ends, i.e., need not be threaded into the sheave system from one end or the other as if threading a needle.

The hoisting mechanism disclosed in each of the prior art patents referred to above includes av drive sheave and tension sheave combination on a first axis, and also includes an idler sheave-pressure sheave combination on a second axis (and, in some cases, third axis) that is associated with the drive sheave-tension sheave combination. The cable is wound around the drive sheave to the idler sheave to the tension sheave in threading the cable through the sheave system,

thereby allowing the hoisting mechanism to ascend and descend the cable. When the hoisting mechanism is resting onthe ground, the idler sheave-pressure sheave combination is manually movable into and out of operating relation with the drive. sheave '(by virtue of being mounted on a pivotable arm structure) to selectively open and close the sheave system s breach so that cable can be threaded or unthreaded with the sheaves. Such a sheave system depends on the weight of the swing stage scaffold or elevator cage, i.e., of the hoisting mechanism and associated structure, to maintain the idler sheave and pressure sheave in operative relation with the drive sheave and tension sheave as the mechanism ascends and descends the cable. I I

In recent years, and because of the increasing widespread use of hoisting mechanisms, certain governmental bodies have passed legislation that set weight support standards which suchhoisting mechanisms must meet under various operating conditions. From the standpoint of such new weight support standards, therefore, the idler sheave type power driven hoisting mechanisms of the prior art referred to above may prove undesirable under certain operating conditions. Further, and from an efficiency of operation standpoint, and particularly at high weight loads, it is desirable that hoisting mechanisms of the type shown in the prior art referred to above stop on the cable as quickly as possible oncepower to the drive sheave is cutoff when descending the cable to prevent jockeying of the mechanism to the desired stopping point. That is, it is highly desirable there be as little drift as possible as the hoisting mechanism descends the' cable once stoppage is desired at'an intermediate point on the cable above ground level. Of course, there is no problem with drift as the hoisting mechanism ascends the cable for the reason that its'weight prevents drifting up the cableonce power is cut off, but as the mechanism descends the cable its weight tends to cause it to drift down the cable once power is cut off and the greater theweight load the worse the drift problem is.

Therefore, it has been one objective of this invention to provide a power driven hoisting mechanism adapted to ascend and descend a freely hanging cable that includes a primary drive sheave and a secondary drive sheave and, in combination therewith, a pressure wheel assemblythat provides an open breach type sheave system for the hoisting mechanism.

It has been another objective of thisinvention to provide a power driven hoisting mechanism adapted to ascend and descend a freely hanging cable wherein drift FIG. 1 is a general side elevational view-of a swing stage scaffoldls stirrup schematically depicting the hoisting mechanism of this invention mounted thereon;

FIG. 2 is a side view of the hoisting mechanism of this invention shown in the closed breach (i.e., ascending and descending) position;-

FIG. 3 is a view similar to FIG. 2 but showing the hoisting mechanism in the open breach (i.e., cable threading) position;

FIG. 4 is a top and partially cut away view of the hoisting mechanism of this invention; and

FIG. 5 is a cross-sectional view showing the drift brake for the hoisting mechanism.

THE GENERAL STRUCTURAL ENVIRONMENT Referring to the drawings, in the general side elevational view of FIG. 1 there is shown the hoisting mechanism of this invention mounted on an A-shaped stirrup or frame 11; such is particularly adapted for use with a swing stage scaffold. Under normal use conditions with a swing stage scaffold, two such hoisting mechanism l0-stirrup 11 structures are used with one being located at each end of a plank or scaffold for raising and lowering that scaffold relative to ground level. The A-shaped stirrup 11 includes a scaffold support member 12 on which one end of the plank scaffold l3 rests, and brackets 14 which receive back rest boards 15. The scaffold 13, of course, is for standing on by the workmen, and the back rest boards 15 are for preventing the workmen from falling backwards off the scaffold as they do thfi job required on the building face 16 or other work surface.

A freely hanging single strand of cable 17 (which provides vertical support to the hoisting mechanism 10, and, hence, to the stirrup 11') is threaded in operative engagement with the hoisting mechanism and extends from the top to the bottom of the stirrup, the mechanism 10 being adapted to climb up and down the cable so as to raise and lower the stirrup rela'tive to ground level. A safety lock 18 (more fully described in Allenbaugh US. Pat. No. 2,931,466) for the cable 17 is mounted at the apex of the stirrup. Cable guides 19 (fixed to the stirrup 1 1 through pieces 21 welded to the stirrup 11) are located above the safety lock 18, between the safety lock and the hoisting mechanism 10, and below the hoisting mechanism, to assist in guiding the cable into and out of the safety lock, and into and out of the hoisting mechanism, as the mechanism ascends and descends the cable. v

The hoisting mechanism 10 is operated by a power unit 22,e.g., a heavy duty drill, fixed to a plate 23 that is welded to the stirrup 11. The power unit 22 is electrically energized through lead 24, the unit being operated by trigger switch 25. A reversing switch (not shown) on the power unit 22 allows the main drive shaft 26 for the hoisting mechanism 10 to be driven clockwise or counterclockwise, thereby causing the mechanism to be positively driven both in its ascent up the cable 17 and its descent down the cable. Therefore,

'4 it is by means of the hoisting mechanism 10 itself, as driven by the power unit 22, that the stirrup frame 11 (and, hence, the swing stage scaffold) ascends and descends the freely hanging cables 17.

THE HOISTING MECHANISM The hoisting mechanism 10 is particularly illustrated in FIGS. 2-4. It is mounted to the stirrup i1 by virtue of being fixed to backup plate 28, the backup plate being welded to cross members 29, 30 of the stirrup.

One of the main components of the hoisting mechanism is sheave system 3l,'see particularly FIG. 4. The sheave system includes a primary drive sheave 32 and a secondary drive sheave 33, both drive sheaves being rotatably carried on housing 34. The primary sheave 32 y is rigidly fixed to primary drive shaft 35 (and is carried in bearings 37), and the secondary sheave 33 is rotatably mounted on fixed secondary shaft 38 (and is carried on bearings 40).'The primary drive shaft 35 is rigidly fixed to a primary worm gear 41 located within chamber 43 inside the housing 34. The primary worm gear 41 meshes with main worm gear 42 rigidly fixed to main drive shaft 26 (the main drive shaft 26 is, of course, rotated by the power unit 22). The primary drive shaft 35 includes a primary-drive gear 48 rigidly fixed to it between the worm gear 41 and the primary sheave 32. This primary drive gear 48 cooperates with a pinion gear 49 mounted for free rotation (through pin 50) to the housing 34. The pinion gear 49 meshes with a secondary drive gear Slrigidly fixed to the end 47 of secondary sheave 33. The secondary shaft 38 is rigidly fixed to the housing 34 as at 36.

Note particularly that the primary 48 and secondary 51 drive gears are of the same dimensions, and that the primary 32 and secondary 33 drive sheaves are of the same dimensions and configuration; Thus, power transmitted through the main drive shaft 26 to the primary drive shaft 35 causes the primary drive sheave 32 to be positively driven and also causes the secondary drive sheave 33 to be positively driven (through gear train both at the same circumferential speed. Thisis an important feature of the power driven hoisting mechanism 10 of this invention in that such provides a dual drum type drive for the cable 17 and, under certain operating conditions, may allow for greater weight loads to be raised and lowered by the hoisting mechanism 10. Also, note that the primary 35 and secondary 38 shafts are parallel one to the other and are in fixed relation one to the other, i.e., they cannot pivot relative one to the other. This is also an important feature in that it simplifies the worm gear train 41, 42, and the drive gear train 48, 49, 51, which transmit power from the power unit 22 into the primary 32 and secondary 33'drive sheaves.

A tension sheave 54 is rotatably fixed to the primary sheaves drive shaft 35 such that it is coaxial with the primary drive sheave 32. Note particularly that the diameter of the tension sheaves one groove is slightly greater than the diameter of the primary drive sheaves grooves, see FIG. 4. The tension sheave 54 is held in rotatable relation with the primary drive shaft 35 by being carried on bearings 55 that are fixed to clutch spring housing 56, the clutch spring housing being fixed to the primary drive shaft by bolt 57; this allows the tension sheave 54 to rotate relative to the drive shaft 35, as well as to rotate exactly with the drive shaft, in a manner to be subsequently explained.

A friction clutch plate 58 is interposed between the illustrated in FIG. 4. The clutch plate 58 is mounted coaxially with the primary drive sheave 32 (as the tension sheave 54 is also mounted coaxially with the primary drive sheave 32) by means of bearings 59; this allows the clutch plate to rotate relative to the drive shaft 35, as well as to rotate exactly with the drive shaft, in

a manner to be subsequently explained. Clutch plate springs 63 are positioned in wells in the clutch spring housing 56, the springs being held within the wells by washer 64. Since the clutch spring housing 56 is. fixed to the drive shaft, same rotates with the drive shaft 35 at all times; and since the clutch spring housing is at all times biased toward the primary drive sheave by virtue of the clutch springs 63, same bias the tension sheave into intimate contact with the clutch plate 58 and the clutch plate into intimate contact with the primary drive sheave 32 at all times. The pressure exerted on tension sheave 54 and clutch plate 58 against primary drive sheave 32 by clutch springs 63 can be adjusted by adjusting the bolt 57, thereby assuring that the tension sheave rotates at the same speed as the primary drive sheave when the primary drive sheave is rotating clockwise as will be explained.

A pawl 65, rotatably fixed to arm 66, cooperates with ratchet teeth 67 on the periphery of the clutch plate 58. The arm 66 is fixed to housing 34 so as to support the pawl 65 in spatial relation with the clutch plate 58, the pawl being constantly urged into engagement with the clutch plates ratchet teeth- 67 by virtue of spring 68. Note, as illustrated in FIG. 2, that the clutch plates teeth 67 are designed so that, in cooperation with pawl 65, the clutch plate 58 is permitted to rotate clockwise as the primary drive sheave 32 rotates clockwise. But the pawl 65 engages the clutch plates teeth 67 to prevent the clutch plate 58 from rotating counterclockwise when the primary drive sheave 32 rotates counterclockwise.

When the cable 17 is threaded with the sheave system 31, i.e., with the hoisting mechanism 10, the upper run 71'of the cable enters the sheave system as at 72 and the lower run 73 of cable leaves the sheave system as at 74. The cables upper run 71 enters the sheave system 31 between the .drive sheaves 32, 33, and the lower run 73 leaves the system 31 from between the drive sheaves, see FIGS. 2 and 4. From point 72 of entry to point 74 of departure, the cable 17 is wound first around primary sheave 32, then onto secondary sheave 33, then back to primary sheave 32 (until four loops 75 of cable have been located in the four grooves of each sheave 32, 33), and finally from the secondary sheave 33 onto and around 'the tension sheave 54. Diverter roller 76 (which is mounted for free rotation to housing 34 on pin 77) serves to divert the first loop 75 of cable. 17 from the appropriatev primary sheaves groove to the appropriate secondary sheaves groove to assure that the loop 75a does not contact the cables upper run 71 as it enters the sheave system 31.

.The upper cable run 71 is, of course, maintained inintimate contact with the primary sheave by virtue of the hoisting mechanisms weight. But the lower cable run 72 is pressed into intimate contact with the tension sheave 54 by means of a pressure wheel assembly 78 since the lower run 72 simply hangs free beneath the hoisting mechanism, see FIGS. 2-4. The pressure wheel assembly 78 is comprised of a major pressure wheel 79 6 and three minor pressure wheels 80 rotatably mounted in a holder 81. The pressure wheels 79, 80 must be in intimate contact with the cable around tension sheave '54, i.e., in the closed breach position, for the hoisting mechanism 10 to ascend and descend the cable; if the pressure wheels 79, 80 are in the open breach position the hoisting mechanism will not ascend and descend the cable. The wheels 79, 80 are arranged in the holder 81 so that their working surfaces define an are 82 of about 45 (preferably the arc 82 is at least about 30) and of about the same radius as the arc of the tension sheaves groove, see FIG. 3; this assures intimate contact of the cable 17 with the tension sheave 54 and prevents buckling of the cable on the tension sheave. The wheels 79, 80 are grooved as at 83 to engage or grip the cable 17, see FIG. 4.

As mentioned, the pressure wheels 79, 8t) are movable between a first position at which the breacharea 84 between the primary 32 and secondary 33 drive sheaves is opened so as to allow easy threading and unthreading ofthe cable. 17 around the primary and secondary sheaves, see FIG. 3, and a second position at which the breach area 84 is closed and at which the pressure wheels 79, 80 press the cable against the tension sheave, see FIGS. 2 and 4. The bracket 81 in which the pressure wheels 79, 80 are freely and rotatably mounted (by means of axle pins 85) carries the wheels between the operating or closed breach position (see FIG. 2) and the cable threading or open breach position (see FIG. 3). The bracket 81 is fixed to a leg 86, the leg being adapted to linearly reciprocate within sleeve 87 (the bracket and "leg serve as the pressure wheels holder or mount). The sleeve 87 is fixed to the shaft 38 (which, in turn, is rigidly fixed to housing 34), and this mounts the pressure wheel assembly 78 in operative engagement with the hoisting mechanism 10. A compression spring 88 is captured within the sleeve 87,

and is disposed to continuously bias the pressure wheels 79, 80 toward the closed breach position. The wheels 79, 80 are movable from the closed breach to the open breach position by manually grasping handle grip 89 fixed to motion arm 90, the motion arm 90 being pivotally connected as at 91 to block 92 fixed to the end of shaft 38. The motion arm 90 defines a cam slot 94 adapted .to cooperate with a pin 95fixed to the pressure wheels bracket 81. Upon grasping the handle grip 89 and pivoting same clockwise about pin 91 (as shown in the figures), the side 96 of slot 94 cams against the pin 91 and causes the pressure wheels 79, 80 to retract toward the open breach position, i.e., causes the pressure wheels bracket 81 and leg 86 to retract linearly into the sleeve 87 against the bias of spring 88, compare FIG. 2 to FIG. 3.

In the retracted or open breach position, and since the, pressure wheels 79, 80 are continually biased toward the closed breach or operating position by spring 88, the pressure wheels 79, 80 must be manually restrained against movement back into operating engagement with thetension sheave 54. This is a safety feature in that it prevents actual or attempted operation of the hoisting system with the pressure wheels 79, 80 out of engagement with tension sheave 54.

A release arm 103 is pivotally mounted to shaft 38 through bracket 104 and pin 105, one end of the re lease arm being pivotally connected to the lock pin 101, see FIGS. 2 and 4. The release arm 103 is spring 106 biased in the clockwise direction as illustrated in FIG. 4. Hence, the lock pin 101 is continually biased.

toward the pressure wheels leg 86 so that as the pressure wheels leg is extended from the sleeve 87 the lock pin 101 seats in hole 102 in that leg (when same are coaxially located) to lock or restrain the pressure wheels 79, 80 into engagement with the tension sheave 54. This assures that the pressure wheels 79, 80 are locked in engagement with the tension sheave 54 during operation of the hoisting mechanism. To release the pressure wheels 79, 80 so that same can be manually moved into the cable threading or open breach position, release finger 103 need merely be depressed in the counterclockwise direction against spring 106 so that pin 101 is withdrawn out of the hole 102 in the pressure wheels leg 86, see FIG. 4.

In use, and when the primary 32 and secondary 33 drive sheaves are driven by main drive shaft 26 in a manner which causes the hoisting mechanism 10 to ascend the cable 17, i.e., clockwise as viewed in the figures, the cable is wound onto the primary drive sheave from the upper run 71 and is played out from the tension drive sheave 54 into the lower run 73. The cable 17 is maintained in firm contact with the tension sheave 54 by the pressure wheel assembly 78. This aids the cable 17 in remaining under tension asit passes back and forth between the primary 32 and secondary 33 drive sheaves, and as it passes around the tension sheave 54. The tension of cable 17 is also due to the fact that the diameter of tension sheave 54 is greater than the diameter of primary 32 and secondary 33 drive sheaves, the tension sheave therefore being rotated at the same rpm as the drive sheaves but at a slightly greater peripheral speed so as to create a slight slippage between the tension sheave and the clutch plate 58 and/or between the primary drive sheave and the clutch plate. The tension applied to the cable 17 within the sheave system 31 in this manner, which is desirable, is dependent upon the adjustment of the clutch springs 63 by which the opposed faces of the primary sheave 32 and tension sheave 54 are held in yielding contact with the faces of clutch plate 58.

When the primary drive sheave 32 is rotated in a direction that allows the hoisting mechanism 10 to de scend the cable 17, i.e., counterclockwise as viewed in the figures, the clutch plate 58 is held stationary by the pawl 65 which prevents rotation of the tension sheave 54 by the primary drive sheave. The cable 17 is withdrawn by the primary 32 and secondary 33 drive sheaves. over the fixed tension sheave 54, and as the cable is withdrawn it is thereby held under firm tension. It will, therefore, be seen that the cable 17 is always maintained in firm contact with the primary 32, secondary 33 and tension54 sheaves, i.e., and in tension with the sheave system 31, with the aid of pressure wheel assembly 78 and as the hoisting mechanism 10 ascends and descends the cable 17.

A drift brake 1 11 is provided in combination with the sheave system 31 and power unit 22 to severely limit downward drift of the hoisting mechanism 10 once power is cut off on descending the cable 17, see FIG. 1. The drift brake l 1 1 is operatively connected with the main drive shaft 26 for the sheave system 31, that drive shaft being connected with the power unit 22, see FIG. 5. The main drive shaft 26 is in two sections, namely, upper part 26a and lower part 26b, which are coupled together by joint 109 just above the drift brake. This joint 109 is enclosed by boot 110. This joint 109 structure allows the power unit .22 to be removed from mounted relation with the stirrup 11 so that it can be used for other purposes if desire The drift brake 111 is an' 'electro-magnetic type device and functions by aiding in stopping rotation of the main drive shaft 26 once the-trigger switch 25 to the power unit 22 has been released by the operator. The drift brake 111 includes a donut shaped magnetic coil 1 15 positioned inter'iorally of housing 1 16, the coil surrounding the main drive shaft 26 and being adapted to cooperate with an upper brake plate 112 and a lower brake plate 114. Tube 107 encloses wire leads to the coil which are, of course, interconnected with an electric power source.

The upper plate 112 is floatably mounted on the.

main drive shaft 26 but is keyed to the shaft 26 so that it rotates therewith; this plate 112 carries upper and lower brake shoes 113a, 1,13b.The lower plate 114 is I slidable on the main drive shaft 26 so that the shaft 26 can rotate relative thereto. The plates 112, 114 are enclosed in housing 116 that surrounds the main drive I shaft 26 and the coil 115, the housing 116 being fixed on top housing 34 of the sheave system 31 provided with ports 118 through the drive shaft 26 freely passes. The lower plate 114 is spring loaded or biased upward toward the 'housings ceiling 117 by compression spring 119, thereby always tending to press or sandwich the brake shoes 113a, 113b between the upper surface of plate 114 and the underside of housings ceiling 117.

In use, such brake 111 components coact to quickly stop rotation of drive shaft 26 (because of the high friction between brake shoes 113a, l13b and the coacting surfaces 114, 117 as caused by the compression spring 119) when power is cut off the power unit 22 and, hence, to the magnetic coil 1 15, through release of trigger switch 25. Alternatively, when the trigger switch 25 'is energized so that the power unit 22 rotates main drive shaft 26, the magnetic coil is also energized so that the magnetic field generated attracts the plate 114 toward the coil, the attraction force of the magnetic field overcoming the repulsion force of spring 119. This releases the brake shoes 1130, 113b from intimate contact with ceiling 117 and plate 114, and allows the drive shaft 26 to rotate and drive the sheave system 10.

Having described in detail the preferred embodiment of my invention, what I desire to claim and protect by Letters Patent is:

1. A hoisting mechanism for ascending and descending a freely hanging, single strand of cable, comprising drive sheaves being urged into intimate contact with the respective faces of said friction clutch,

and beingone-way rotation means mounted in cooperative relation with said friction clutch such'that said friction clutch can rotate in one direction only,

a pressure wheel assembly mounted on said housing,

said pressure wheel assembly being movable independently of said primary and secondary drive sheaves between a closed breach position where a pressure wheel intimately presses the cable against said tension sheave so that the hoisting mechanism is adapted to ascend and descend the cable and an open breach position where the pressure wheel is retracted from the tension sheave to open the breach area of the sheave system for threading and unthreading with the cable, 7

spring means constantly urging said pressure wheel toward the closed breach position, and

a handle pivotally mounted to said housing and connected to a holder for said pressure wheel, the

manually pivoting of said handle serving to retract said pressure wheel into the open breach position.

a tension sheave coaxially mounted with, andfor rotation relative to, one of said drive sheaves,

a double faced friction clutch interposed between said tension sheave and that drive sheave with which it is coaxially mounted, said tension and drive sheaves being urged into intimate contact with the respective faces of said friction clutch,

one-way rotation means mounted in cooperative relation with said friction clutch such that said friction clutch can rotate in one direction only,

a pressure wheel assembly mounted on said housing,

, said pressure wheel assembly being movable independently of said primary and secondary drive sheaves between a closed breach position where a pressure wheel intimately presses the cable against said tension sheave so that the hoisting mechanism is adapted to ascend and descend the cable and an open breach position where the pressure wheel is retracted from the tension sheave to open the breach area of the sheave system for threading and unthreading with the cable, and

a lock device continually biased toward locking engagement with a holder for said pressure wheel, said lock device serving to lock said pressure wheel in the closed breach position when said pressure wheel is in the closed breach position, and-said lock device being manually retractable away from said holder when it is desired that said pressure wheel be manually pivoted to the open breach position. 

1. A hoisting mechanism for ascending and descending a freely hanging, single strand of cable, comprising a first drive sheave and a second drive sheave mounted on a housing, said drive sheaves being located on axes substantially parallel one to the other and said drive sheave axes being immobile relative one to the other, power means connected with said drive sheaves for positively rotating each of said first and second drive sheaves, a tension sheave coaxially mounted with, and for rotation relative to, one of said drive sheaves, a double faced friction clutch interposed between said tension sheave and that drive sheave with which it is coaxially mounted, said tension and drive sheaves being urged into intimate contact with the respective faces of said friction clutch, one-way rotation means mounted in cooperative relation with said friction clutch such that said friction clutch can rotate in one direction only, a pressure wheel assembly mounted on said housing, said pressure wheel assembly being movable independently of said primary and secondary drive sheaves between a closed breach position where a pressure wheel intimately presses the cable against said tension sheave so that the hoisting mechanism is adapted to ascend and descend the cable and an open breach position where the pressure wheel is retracted from the tension sheave to open the breach area of the sheave system for threading and unthreading with the cable, spring means constantly urging said pressure wheel toward the closed breach position, and a handle pivotally mounted to said housing and connected to a holder for said pressure wheel, the manually pivoting of said handle serving to retract said pressure wheel into the open breach position.
 2. A hoisting mechanism for ascending and descending a freely hanging, single strand of cable, comprising a first drive sheave and a second drive sheave mounted on a housing, said drive sheaves being located on axes substantially parallel one to the other and said drive sheave axes being immobile relative one to the other, power means connected with said first drive sheaves for positively rotating each of said first and second drive sheaves, a tension sheave coaxially mounted with, and for rotation relative to, one of said drive sheaves, a double faced friction clutch interposed between said tension sheave and that drive sheave with which it is coaxially mounted, said tension and drive sheaves being urged into intimate contact with the respective faces of said friction clutch, one-way rotation means mounted in cooperative relation with said friction clutch such that said friction clutch can rotate in one direction only, a pressure wheel assembly mounted on said housing, said pressure wheel assembly being movable independently of said primary and secondary drive sheaves between a closed breach position where a pressure wheel intimately presses the cable against said tension sheave so that the hoisting mechanism is adapted to ascend and descend the cable and an open breach position where the pressure wheel is retracted from the tension sheave to open the breach area of the sheave system for threading and unthreading with the cable, and a lock device continually biased toward locking engagement with a holder for said pressure wheel, said lock device serving to lock said pressure wheel in the closed breach position when said pressure wheel is in the closed breach position, and said lock device being manually retractable away from said holder when it is desired that said pressure wheel be manually pivoted to the open breach position. 